This application claims the priority benefit of Taiwan application Ser. No. 89125183, filed Nov. 28, 2000.
1. Field of Invention
The present invention relates to a chemical-mechanical polishing method. More particularly, the present invention relates to a chemical-mechanical polishing method for polishing a copper layer.
2. Description of Related Art
Chemical mechanical polishing is one of the global planarization methods known and adopted in an ultra-large scale integration (ULSI) integrated circuit fabrication. The method is a planarization technique based on a mechanical polishing theory using an abrasive means. The abrasive means is then supplied with an appropriate reagent to planarize by polishing a non-planar surface profile of the wafer.
In chemical-mechanical polishing, the reagent usually refers to a slurry, which is mainly a colloidal solution of silica or dispersed alumina solution. The hard silica or alumina particles inside the slurry provide the necessary abrasion for polishing a silicon chip.
FIGS. 1A and 1B are schematic cross-sectional views showing the method of planarizing a copper layer by conventional chemical-mechanical polishing. As shown in FIG. 1A, a substrate 100 having a wide opening 102a and several densely distributed narrow openings 102b is provided. A copper layer 104 is formed to completely cover the substrate 100 and fill the wide opening 102a and the narrow openings 102b. Since the upper surface of the copper layer 104 is exposed to air, a copper oxide layer 106 is formed over the copper layer 104.
During the method of forming a damascene structure, the copper oxide layer 106 as well as any excess portion of the copper layer 104 above the substrate 100 must be removed. However, as the copper oxide 106 has an unusual hardness, it would require a long polishing time to remove the copper oxide layer 106 if conventional slurry were used in the polishing method. This leads to a drop in productivity. To speed up the polishing rate, slurry 108 having a high concentration of suspended particles has been conventionally used to polish the copper oxide layer 106 and the copper layer 104. The suspended particles made up about 5% concentration of the slurry 108.
Since the copper oxide layer 106 and an excess portion of the copper layer 104 are simultaneously removed using the slurry 108 highly concentrated with the suspended particles, the wide opening 102a forms a dish surface as shown in FIG. 1B. Such dishing problem also occurs in the narrow opening 102b. In addition, erosion of the dielectric layer in the substrate 100 occurs in a region densely distributed with narrow openings 102b, leading to other problems.
Accordingly, the present invention provides a chemical-mechanical polishing method for copper to prevent dishing problem due to the use of an aqueous solution containing a high concentration of polishing particles, while an erosion of a substrate in a densely distributed copper wire region is avoided.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a chemical-mechanical polishing method for copper. First, an aqueous solution having a high concentration of polishing particles/chelating agent is used to polish away a copper oxide layer naturally formed at the upper surface of a copper layer. A polishing slurry having either a low concentration of polishing particles/chelating agent or free of polishing-particle/chelating agent is then used to continue the polishing of the copper layer.
This invention also provides an alternative chemical-mechanical polishing method for copper. First, the polishing slurry and the aqueous solution having a high concentration of polishing particles/chelating agent are mixed in situ to remove the copper oxide layer at the top surface of the copper layer. After the copper oxide is removed, the polishing slurry having a low concentration of the polishing particles/chelating agent or free of the polishing-particles/chelating agent is used in the polishing step to polish the copper layer.
One major aspect of this invention is an initial increase in the polishing rate of the copper oxide layer, while the dishing of the copper layer and the erosion of the substrate surface that occurred are minimized.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.